© 2016, The Author(s). Background: One of the most important risk factors for coronary artery disease is physical inactivity. Health psychological research demonstrates the importance of planning for behaviour change success. Consequently, a health action process approach (HAPA) model-based design to support the uptake of physical activity was initiated for the first time in an acute cardiac ward. Methods: For impact evaluation, a control group (CG) and an intervention group (IG) of coronary artery disease patients were compared in a controlled longitudinal study. Baseline assessment included socio-demographic variables, intentions regarding physical activity, and actual physical activity prior to the coronary artery disease event. Follow-up data were collected 2 and 6 months after discharge. Results: In total, 193 patients participated in this controlled longitudinal study (63 ± 9 years; CG: N = 78; IG: N = 115). The IG reported a higher increase in physical activity (p <0.05), intentions, and coping planning (p <0.05), and also in action planning and control (p <0.01) 2 months after discharge. Both CG and IG increased their physical activity 6 months after discharge to the point of no significant difference (p = 0.664). Conclusions: A HAPA model-based health psychological intervention on an acute cardiac ward is able to increase patients’ physical activity over the short term. However, integration of follow-up interventions (preferable in cardiac rehabilitation settings) would be necessary to support sustained physical activity.